In 1755 LeRoy passed the discharge of a Leyden jar through the orbit of a man who was blind from cataract and the patient saw "flames passing rapidly downwards." Ever since, there has been a fascination with electrically elicited visual perception. The general concepts of electrical stimulation of retinal cells to produce these flashes of light or phosphenes has been known for quite some time. Based on these general principles, some early attempts at devising a prosthesis for aiding the visually impaired have included attaching electrodes to the head or eyelids of patients. While some of these early attempts met with some limited success, the phosphenes which were perceived were unfocused and could not approach actual vision restoration or simulation because of the gross, unfocused stimulation of the patient's eye or the optical nerve.
As intraocular surgical techniques advanced, it became possible to apply a more focused stimulation on small groups of, and even on individual, retinal cells to generate focused phosphenes. This focused phosphene generation opens the possibility of true simulated vision generation by an implanted prosthesis within the eye itself. This has sparked renewed interest in developing methods and apparatuses to aid the visually impaired. Specifically, great effort has been expended in the area of focused stimulation of retinal elements proximal to degenerated photoreceptors which occur in certain forms of retinal blindness and which affect millions of people worldwide. However, while the surgical techniques had advanced to the point of allowing access to the retina, and while the structure and function of the retinal cells was understood, a complete understanding of the individual processes and mechanisms of simulated vision through retinal cellular stimulation was not completely understood in these early days.
What was known about the structure of the retina is that the retinal basement membrane 10 (see FIG. 1) is at the surface of the retina, above the axons 11 which emanate from the retinal ganglion cells 12. These axons 11 which emanate from the retinal ganglion cells eventually come together and form the optic nerve (not shown) which projects to the brain. Beneath the retinal ganglion cells 12 are nerve cells involved in intermediate signal processing, such as amacrine cells 13, bipolar cells 14, interplexiform cells 15, and horizontal cells 16. At the back or outer layer of the retina are the photoreceptor cells 17. In degenerative diseases of the retina, such as retinitis pigmentosa, the photoreceptor cells 17 degrade, but the other nerve cells remain viable.
Pioneering work by de Juan, Jr. et al. embodied in U.S. Pat. No. 5,109,844 for RETINAL MICROSTIMULATION which issued May 5, 1992, provided the teaching for a method for stimulating the still viable retinal cells as well as for an apparatus for practicing this method, said teachings and disclosure being hereby incorporated by reference. As taught by de Juan, Jr. et al. '884, a focused stimulation of the retinal ganglion cells 12 could produce focused phosphenes which, if stimulated by an array apparatus, could simulate vision. De Juan, Jr. et al. '844 also teaches that the stimulation current capable of penetrating the retina to an excitation depth of approximately 30 micrometers is sufficient to depolarize the ganglion cells and evoke an action potential therefrom, the patient's perception of which is a focused phosphene. While de Juan, Jr. et al. '844 does not dwell on the stimulation waveform, this patent does teach that the waveform should preferably have an amplitude of not greater than about 0.3 to 3 milliampere and be biphasic having pulse duration of about 0.1 to about 2 milliseconds per phase, with a frequency of about 50 to 100 hertz.
Since the axons from the ganglion cells traverse the surface of the retina on their way to form the optic nerve as discussed briefly above, it is recognized that to produce a focused phosphene, inadvertent stimulation of axons from distant ganglion cells which lie adjacent the target ganglion cells must be avoided. An inadvertent stimulation of adjacent axons from distant ganglion cells results in the perception of a wedge of light as opposed to a focused point of light and makes clear simulated vision through a retinal prosthesis difficult to obtain.
One method of focused retinal cell stimulation which attempts to avoid the problem of inadvertent adjacent axon stimulation is described in Edell et al., U.S. Pat. No. 5,411,540, issued May 2, 1995, for a METHOD AND APPARATUS FOR PREFERENTIAL NEURON STIMULATION. Edell et al. '540 describes the use of anodic (positive) stimulation to preferentially stimulate retinal ganglia somas while simultaneously avoiding unwanted stimulation of nearby unrelated axons to produce a focused phosphene. This reference describes that this positive pulse scheme requires a waveform pulse of duration between about 1 microsecond and about 500 microsecond having an amplitude of between about 1 microampere and about 500 microampere at a frequency of up to 1 kHz. Edell et al. '540 also teaches that the particular geometry of the electrode has a direct impact on the effectiveness of its method of stimulation of the ganglia soma and on the inadvertent and undesired stimulation of unrelated superficial axons from distant retinal ganglia soma. Edell et al. '540, therefore, requires specific geometry electrodes to perform the focused stimulation. However, the added complexity resulting from the criticality of placement and specific geometry of the electrodes, as well as the potential cellular effects of anodic (positive) stimulation and likely inadvertent stimulation of unrelated axons anyway, make this approach less desirable.